1. Field of the Invention
This invention relates to the field of microprocessors and, more particularly, to the dispatching of floating point exchange instructions within microprocessors.
2. Description of the Related Art
Superscalar microprocessors achieve high performance by executing multiple instructions per clock cycle and by choosing the shortest possible clock cycle consistent with the design. As used herein, the term "clock cycle" refers to an interval of time accorded to various stages of an instruction processing pipeline within the microprocessor. Storage devices (e.g. registers and arrays) capture their values according to the clock cycle. For example, a storage device may capture a value according to a rising or falling edge of a clock signal defining the clock cycle. The storage device then stores the value until the subsequent rising or falling edge of the clock signal, respectively. The term "instruction processing pipeline" is used herein to refer to the logic circuits employed to process instructions in a pipelined fashion. Generally speaking, a pipeline comprises a number of stages at which portions of a particular task are performed. Different stages may simultaneously operate upon different items, thereby increasing overall throughput. Although the instruction processing pipeline may be divided into any number of stages at which portions of instruction processing are performed, instruction processing generally comprises fetching the instruction, decoding the instruction, executing the instruction, and storing the execution results in the destination identified by the instruction.
Microprocessors are configured to operate upon various data types in response to various instructions. For example, certain instructions are defined to operate upon an integer data type. The bits representing an integer form the digits of the number. The decimal point is assumed to be to the right of the digits (i.e. integers are whole numbers). Another data type often employed in microprocessors is the floating point data type. Floating point numbers are represented by a significand and an exponent. The base for the floating point number is raised to the power of the exponent and multiplied by the significand to arrive at the number represented. While any base may be used, base 2 is common in many microprocessors. The significand comprises a number of bits used to represent the most significant digits of the number. Typically, the significand comprises one bit to the left of the binary, and the remaining bits to the right of the binary. The bit to the left of the binary point is not explicitly stored, instead it is implied in the format of the number. Generally, the exponent and the significand of the floating point number are stored. Additional information regarding the floating point numbers and operations performed thereon may be obtained in the Institute of Electrical and Electronic Engineers (IEEE) standard 754.
Floating point numbers can represent numbers within a much larger range than can integer numbers. For example, a 32 bit signed integer can represent the integers between 2.sup.31 -1 and -2.sup.31, when two's complement format is used. A single precision floating point number as defined by IEEE 754 comprises 32 bits (a one bit sign, 8 bit biased exponent, and 24 bits of significand) and has a range from 2.sup.-126 to 2.sup.127 in both positive and negative numbers. A double precision (64 bit) floating point value has a range from 2.sup.-1022 and 2.sup.1023 in both positive and negative numbers. Finally, an extended precision (80 bit) floating point number has a range from 2.sup.-16382 to 2.sup.16383 in both positive and negative numbers.
The expanded range available using the floating point data type is advantageous for many types of calculations in which large variations in the magnitude of numbers can be expected, as well as in computationally intensive tasks in which intermediate results may vary widely in magnitude from the input values and output values. Still further, greater precision may be available in floating point data types than is available in integer data types.
Floating point data types and floating point instructions produce challenges for the microprocessor designer. Floating point instructions are typically executed by a specialized unit designed to perform floating point operations. Accordingly, the microprocessor must identify floating point instructions and dispatch those instructions to a floating point instruction unit. Floating point instruction units are typically designed to execute one floating point instruction at a time.
Floating point instructions are typically stack based instructions. The instructions are designed to operate on data stored on the top of a register stack. Because each instruction uses the top-of-stack register, register dependencies exist between floating point instructions and the floating point instructions must be executed in a serial fashion. When a register other than the top of the register stack is the desired operand for a floating point instruction, a floating point exchange (FXCH) instruction is executed. The floating point exchange instruction exchanges the contents of a specified floating register with the contents of the top-of-stack register. The floating point instruction is then executed using the top-of-stack register. Unfortunately, the execution of a floating point instruction on a register other than the top-of-stack requires two floating point instructions. As mentioned above, only one floating point instruction is typically executed per clock cycle. Accordingly, executing a floating point instruction on a register other than the top-of-stack register requires at least two clock cycles to perform.